1. Field of the Invention
The field of the invention is that of bipolar transistors and especially vertical structure heterojunction bipolar transistors in which the current is conveyed perpendicularly to the surface of the substrate. These transistors are used for microwave applications.
A bipolar transistor is constituted by a stack of layers of semiconductor materials, of which at least one surface layer is etched in a mesa formation, namely a raised or embossed pattern. FIG. 1 illustrates a standard structure of a heterojunction bipolar transistor or HBT comprising a substrate S, a sub-collector SC, a collector C, a base B and an emitter E. Conventionally, a contact CE is made on the surface of the emitter by the superimposition of matched layers. Laterally, two contacts CB1 and CB2 are also made in the collector on either side of the base.
This type of vertical structure raises a problem of electron/hole recombination at the free surfaces S1 and S2 shown in FIG. 1. These recombinations play a major role in the deterioration of current gain.
The smaller the size of the transistors, the greater is this phenomenon, with the surface recombination phenomena playing a greater role. Consequently, the microwave applications of such structures are heavily penalized by this problem. Indeed, for microwave operation, a bipolar transistor consists of several elementary transistors (also called fingers) positioned in parallel. To limit the base resistance, it is necessary to limit the width of the emitter and therefore the width of each finger. Typically, the width of a finger may be in the range of 2 xcexcm for applications using frequencies below 100 GHz while the length of the finger of the emitter may be about 30 microns.
2. Description of the Prior Art
Certain solutions have already been brought to bear on this problem, especially in the context of HBT transistors made with GaAlAs/GaAs type materials. Thus, it has been proposed to introduce a passivation layer by leaving, on the surface between the emitter and the base, a semiconductor material with a wide gap that is very thin and therefore depopulated. This prevents the appearance of recombinant electrical defects at the base/passivation interface. More specifically, FIG. 2 describes a structure in which the emitter E has a special architecture obtained by the partial etching of a layer of GaAlAs deposited on a layer of GaAs constituting the base B. Two contacts CB1 and CB2 are made on the emitter layer on the surfaces Sxe2x80x21 and Sxe2x80x22. Then, by an appropriate treatment, these contacts may get diffused on the thickness eo. In this configuration, the electron/hole recombinations in the base, which previously enjoyed favorable conditions at the free surface of the base, are limited owing to the emitter thickness eo maintained above the base. The problem of this type of structure however lies in their poor stability inasmuch as the diffusion achieved for the contacts cannot be entirely controlled and may therefore continue to undergo changes with time and temperature.
This is why the invention proposes a heterojunction bipolar transistor in which the surface recombinations are minimized through electrically insulating elements located on a part of the base and in direct contact with the flanks of the emitter mesa.
The invention can be applied to structures in which the mesa is an emitter mesa as well as to structures in which the mesa is a collector mesa.
More specifically, an object of the invention is a heterojunction bipolar transistor based on III-V semiconductor materials comprising a collector, a base and an emitter and having a mesa located on the base, said transistor furthermore comprising electrically insulating elements in contact with the free surface of the base and in contact with the flanks of the mesa, the width of said elements being of the same magnitude as the width of said mesa.
According to one variant of the invention, the mesa is an emitter mesa constituted by p doped Gaxln1xe2x88x92xP, the base is constituted by GayIn1xe2x88x92yAs, the electrically insulating elements being formed by GaxIn1xe2x88x92xP containing boron ions.
Advantageously, the mesa may comprise, on the surface, an ohmic contact layer made of refractory metal of the TiWSi, WN, TiW or other type.
According to one variant of the invention, the bipolar transistor comprises a passivation layer.
According to one variant of the invention, the bipolar transistor comprises a heat sink in the form of a bridge lying on the one hand on the mesa and on the other hand on the substrate.
An object of the invention is also a method for making a heterojunction bipolar transistor based on III-V semiconductor materials comprising the epitaxial growth of semiconductor layers including a p type (or n type) doped layer constituting the base, between two n type (or p type) doped layers constituting the emitter and the collector, wherein said transistor further comprises:
the ion implantation, through a mask with a width l, of electrically insulating ions in the said upper layer, located above the base layer;
the etching through a mask with a width L, greater than the width l, of the layer that has been made to be locally electrically insulating, so as to define electrically insulating elements (61) on either side of doped semiconductor elements (40).
Advantageously, the layer called the upper layer and the base layer have different types of behavior with respect to the ion implantation. Only the layer called the upper layer has its electrical properties modified by said ion implantation.